The present invention relates to an electric safety switch and more particularly to a pressure switch for electric motors of apparatus and vehicles stopping of which must be ensured under any condition, even in case of blocking of the switch itself.
Switches of this type are already known in the art: U.S. Pat. No. 5,304,753 for example depicts a safety press switch of the mentioned type.
Said switch comprises a contact-holding slide which is slidable under the thrust of an activation button receiving current from a terminal which is always alive and transmits said current to the terminal of an electric motor or alternatively to the terminal of a braking device. More specifically, when the button is in a rest condition the slide powers the brake terminal while the motor terminal is disconnected, and the opposite occurs when the button is in a work condition. A counter-spring bucks displacement of the button keeping it normally in a rest condition. The slide consists of two parts electrically in contact with one another but susceptible of mechanical separation: the slide, during its stroke, opens the contact with the brake terminal and, on reaching its end of stroke, closes the contact with the motor.
It may happen that repeated opening and closing cycles of the motor contact cause the switch to get blocked by welding between the contact point on the slide and the contact point on the terminal, due to the electric arc created between said points at each opening and closing movement of the contact.
Without an appropriate safety device, this drawback, since the motor is maintained always powered, constitutes a great risk for the safety of the operator assigned to the machine or for the vehicle driver and also for the safety of the apparatus itself.
In the switch of the described type safety consists in that, in case of welding between the contact points of the motor contact, when pressure on the activation button is released, the two slide parts separate under the thrust of the counter-spring, thereby breaking the electric contact between the portion welded to the motor terminal and the portion always alive. The slide portion coming back to the rest position restores the electric contact with the, brake terminal and blocks the kinematic motion of all movable parts, preventing any reuse of the switch.
In a different type of switch the circuit for powering the motor terminal contemplates two contact points placed on either side of the slide and disposed at a mutual distance along the stroke of the slide: the slide during its stroke first closes a first contact on which no sparking occurs because the motor terminal is still disconnected, and subsequently closes a second contact of true connection with the motor. In case of welding between the contacts, said welding only takes place on the second contact: on occurrence of said welding the slide, during its return stroke, tilts relative to its normal movement axis because it is retained on the side where the second welded contact is present and free on, the opposite side where the first contact is present, which first contact opens without difficulty interrupting power supply to the motor. However, this inclination of the slide causes blocking of the kinematic motion of the movable parts making the switch unusable.
All these switches have serious limits and drawbacks.
A first drawback arises from the fact that this type of safety asks for a very long stroke of the activation button and the slide and this constitutes an annoying dimensional bulkiness and a factor of mechanical weakness of the switch.
A different drawback resides in that this type of structure needs structurally weak components from an electric point of view as well, since the components must maintain reduced physical sizes in order not to have a switch which is too bulky and heavy.
Another drawback is due to the fact that these switches do not contemplate the presence of any protection device against a too prolonged continuous use of same. As a consequence, the switch that badly withstands overheating due to such a use, is often subjected to failure with or without occurrence of welding of the contact points on the terminals and it does not even protect the motor from the consequences of such a use, involving economic charges for servicing, repair and replacement of damaged parts in the apparatus.
Now the Applicant has become aware of the fact that the listed problems could be solved with a switch of a new type capable of simultaneously ensuring full safety, functional qualities, reliability and low cost. In particular, the Applicant has sensed that a solution could be found in a particular combination between the switch architecture, the position of the contacts and the features of the employed materials.